This invention relates to plasma generation equipment, and is particularly directed to probes for detecting the current and voltage of RF electrical power that is being supplied to an RF plasma chamber.
In a typical RF plasma generator arrangement, a high power RF source produces an RF wave at a preset frequency, i.e., 13.56 MHz, and this is furnished along a power conduit to a plasma chamber. Because there is typically a severe impedance mismatch between the RF power source and the plasma chamber, an impedance matching network is interposed between the two. There are non-linearities in the plasma chamber, and because of these and because of losses in the line and in the impedance matching network, the output of the RF generator does not all reach the plasma chamber. Therefore, it is conventional to employ a probe at the power input to the plasma chamber to detect the voltage and current of the RF wave as it enters the plasma chamber. By accurately measuring the voltage and current as close to the chamber as possible, the user of the plasma process can obtain a better indication of the quality of the plasma. This in turn yields better control of the etching characteristics for a silicon wafer or other workpiece in the chamber.
At the present time, diode probes are employed to detect the amplitude of the current and voltage waveforms. These probes simply employ the diodes to rectify the voltage and current waveforms, and deliver a simple DC metering output for voltage and for current. These probes have at least two drawbacks in this role. Diode detectors are inherently non-linear at low signal levels, and are notoriously subject to temperature drift. The diodes also are limited to detecting the signal peaks for the fundamental frequency only, and cannot yield any information about higher frequencies present in the RF power waveform. This means that for any harmonic information, it is impossible to obtain "harmonic fingerprints" and also that power measurement is not accurate when the waveform is rich in harmonics, as is usually the case in plasma work. In addition to this, it is impossible to obtain phase angle information between the current and voltage waveforms, which also renders the power measurement less accurate.
One proposal that has been considered to improve the detection of RF power has been to obtain digital samples of the voltage and current outputs of a probe, using flash conversion, and then to process the samples on a high-speed buffer RAM. However, this proposal does have problems with accuracy and precision. At the present time, flash conversion has a low dynamic range, being limited to about eight bits. To gain reasonable phase accuracy for plasma customer requirements, it is necessary to reach a precision of about twelve bits, so that a phase angle precision of better than one degree can be obtained at full power. In addition, flash converters require an extremely fast RAM in order to buffer a block of samples before they are processed in a digital signal processor (DSP), and the RAM circuitry is both space-consuming and expensive.